In many applications where a signal is used to provide information in some form, the quality and/or reliability of that information can be degraded by other unwanted signals or noise. This is particularly true of communications systems. To obtain high quality reception, communication systems (including mobile communications, such as Broadcast FM radio or cellular telephone in automobiles, require a strong signal that is not corrupted by noise or distortion). One source of noise and distortion that can severely degrade reception is multipath. Multipath occurs when the transmitted signal arrives at the receiver simultaneously from more than one direction. The multiple paths are generally due to reflections of the transmitted signal from hills, buildings, etc. and can also be the result of atmospheric phenomena. Multipath can cause distortion in the amplitude, phase and frequency of the received signal, which can result in deep signal strength fades, overlapping data, noise, etc.
One approach well known in the art for reducing the effects of multipath and interference is the adaptive array (Widrow, B. & others, "Adaptive Antenna Systems", Proceedings of the IEEE, Vol. 55, No. 12, December 1967, pp. 2143-2159; Treichler, John R. and Agee, Brian G., "A New Approach to Multipath Correction of Constant Modulus Signals", IEEE Transactions on Acoustics. Speech and Signal Processing, Vol. ASSP-31, No. 2, April 1983, pp.459-471; Monzingo, Robert A. and Miller, Thomas W., Introduction to Adaptive Arrays, John Wiley & Sons, New York, 1980; U.S. Pat. Nos. 4,736,460, Multipath Reduction System by Kenneth Rilling, 4,797,950, Multipath Reduction System by Kenneth Rilling, and 4,752,969, Anti-Multipath Signal Processor by Kenneth Rilling). The performance of adaptive arrays can depend strongly on the gain of the adaptive loops, however, in some adaptive arrays the gain of the adaptive loops depend on the level of the input signals to the adaptive array. In some applications, such as mobile communications (i.e., FM radio in automobiles), the dynamic range of the input signals of an adaptive array can be very large, causing a large change in the loop gain. This pushes the adaptive array off its performance peak, resulting in reduced multipath noise and interference reduction.
The steerable null antenna in U.S. Pat. Nos. 4,280,128 (Adaptive Steerable Null Antenna Processor) by Raymond J. Masak and 4,268,829 (Adaptive Steerable Null Antenna Processor with Gain Control) by Herbert F. Baurle and Raymond J. Masak changes the gain of its adaptive loops. The steerable null antenna is used to reject interference which is greater in power than the desired signal. In U.S. Pat. 4,268,829, Baurle and Masak increase the dynamic range of the steerable null antenna by using AGC circuits in the feedback and antenna-correlator signal paths. And in U.S. Pat. No. 4,280,128, Masak reduces the degradation in the "sensitivity" of the steerable null antenna caused by an amplitude imbalance between the two antenna elements caused by multipath or mutual coupling between antenna elements with a special amplitude weighting apparatus.